We have actually validated three Solder Pastes nominally suitable for Hotbar thermocompression bonding on our production line. Two of them exhibited obvious slump after printing through a 0.15 mm-thick stencil, resulting in uneven solder volume distribution during thermocompression. A truly usable Hotbar solder paste must maintain a thixotropic index above 600 Pa·s at room temperature, and soften rapidly without flowing after preheating at 35°C for 10 seconds; otherwise, lateral extrusion of the solder paste and a sharp increase in bridging risk will occur the moment the thermocompression head presses down.

The activity window of laser soldering paste is much narrower than that of traditional Reflow Soldering, because 1064 nm fiber lasers are extremely sensitive to Flux components. Solder pastes containing halogen activators are highly prone to solder popping and spattering under a single pulse. Therefore, we have now fully switched to an organic acid compound system. Although it leaves slightly more rosin residue, it can ensure a continuous molten pool at a scanning speed of 0.8 mm/s and a single-point exposure time of 300 ms, and no gray-black microspots formed by laser-induced oxidation will appear on the solder joint surface.

Many customers have reported insufficient thrust after Hotbar soldering. In fact, the problem often lies in the glass transition temperature setting of the flux carrier resin. Resins with a glass transition temperature below 110°C will soften excessively and be squeezed out from the edge of the pad under the thermocompression head condition of 180°C/2 s, compressing the growth space of the intermetallic compound IMC. We have measured that for Hotbar solder pastes with the glass transition temperature controlled in the range of 122±3°C, the thickness of the Cu6Sn5 layer is stable at 1.8~2.1 μm, and the thrust values are concentrated between 4.2~4.6 N, fully meeting the requirements of automotive-grade connectors.

Laser soldering paste has stricter requirements for the sphericity of metal powder than ordinary solder pastes. If the solder powder with a D50 particle size of 25 μm has a sphericity below 92%, local hot spots are likely to form at the laser focal point, causing some solder to remain unmelted while adjacent areas are already overheated and oxidized. Therefore, all mass production batches now mandatorily include SEM sphericity sampling inspection. Powder images from at least three different positions are sampled per batch, and the average sphericity must be ≥94.3% before release.

We once experienced a drop in Hotbar soldering yield from 99.2% to 94.7% due to a viscosity fluctuation of ±80 Pa·s between solder paste batches. It was later confirmed that this was caused by a batch change of the rosin ester tackifier. Now, it is stipulated that the same batch of tackifier must be used for the same order, and each can of solder paste must undergo a re-test of rotational viscosity at 25°C/1 rpm/60 seconds before delivery. If the data deviates from the nominal value by ±5%, the entire can will be rejected.

Laser soldering paste must be used under nitrogen protection after opening. Once exposed to workshop air for more than 4 hours, an invisible SnO passivation film will form on the surface, directly causing the wetting angle of the first solder joint to be greater than 55°. Therefore, all our laser soldering stations are now equipped with nitrogen flow meters and oxygen content sensors to ensure that the O? concentration in the solder paste contact area is always below 100 ppm.